The demand for hydrogen is predicted to increase sharply in various industrial fields. With such a background, development of a hydrogen sensor used for detection of leakage of hydrogen gas or for measurement of hydrogen gas concentration is in progress.
With respect to the sensor for detection of hydrogen gas, various proposals have been made heretofore. For example, there was proposed a sensor using, as a hydrogen detection element, a thin film of a rare earth metal such as yttrium (Y), lanthanum (La) or the like (the claim of Japanese National Publication of International Patent Application No. 2002-535651).
In this proposal, the change in physical properties of rare earth metal which appears when the rare earth metal is exposed to hydrogen, is utilized for detection of hydrogen. In this sensor, in order to prevent the harmful influence which the rare earth metal receives from the non-hydrogen gas components present together with hydrogen, such as nitrogen, oxygen, ammonia, hydrocarbons and the like, the surface of the rare earth metal film is covered with a protective film composed of, for example, palladium (Pa), platinum (Pt) or an alloy thereof, all having hydrogen permeability.
However, hydrogen-permeable metals such as Pd, Pt and the like expand or contract whenever they absorb or release hydrogen. This expansion and contraction causes mechanical stress, which deteriorates the hydrogen-permeable metal with the lapse of time and generates cracks, etc. Therefore, a hydrogen sensor using a hydrogen-permeable metal as a protective film is inferior in durability.
Further, the hydrogen-permeable metals such as Pd, Pt and the like tend to diffuse into a rare earth metal such as yttrium (Y), lanthanum (La) or the like. As this diffusion proceeds, the hydrogen detectability of the rare earth metal decreases with the lapse of time. Therefore, a hydrogen sensor using a hydrogen-permeable metal as a protective film has a problem in durability.
Furthermore, since Pd or Pt is a very expensive metal, a hydrogen sensor using such a metal as a protective film incurs a high production cost.
The above-mentioned hydrogen sensor has other problems; that is, the structure is complicated and, since the sensor can detect only a low concentration of hydrogen, the application thereof may be limited to warning of hydrogen leakage. Therefore, the hydrogen sensor is unusable in an application of quantitatively determining a high concentration of hydrogen for control of the concentration of the hydrogen gas or in an application of immediately stopping a system in an emergency case.
Thus, development of a hydrogen sensor of good durability and low cost is desired strongly.